It looks like you're using Internet Explorer 11 or older. This website works best with modern browsers such as the latest versions of Chrome, Firefox, Safari, and Edge. If you continue with this browser, you may see unexpected results.

# Clinical Calculations: Module 6: Divided Doses and Reconstituted Medications

## Divided Doses and Reconstituted Medications

### Module 6 – Calculating Divided Doses and Doses of Reconstituted Medications

#### What’s in this module?

There are two problem types in this module: medication given in divided doses and reconstitution of powdered medications.

#### Summary of problem types in this module

Problems will involve reading medication labels and healthcare provider’s orders.  You will calculate the correct amount to give a client of oral, injectable, and intravenous medications.  Assume all questions ask for the amount per dose unless instructed otherwise.

#### Equivalents to know

You should now know all your commonly used equivalents.

#### Rounding rules to know

You will continue to use the rounding rules for numbers >1 and <1.

You will also need to refer to your rounding rules for tablets and capsules.

#### Starting factors and answer units

The SF in most of the problems is the amount of medication the healthcare provider has prescribed.  The dose is usually stated in mg, mcg, or g.

Weight-based medications will use the client’s weight as the SF.

#### Medication Given in Divided Doses

The healthcare provider may prescribe the total amount of a medication to be given in a day (24 hours).  The medication prescription may state that the medication is to be given in a specific number of doses (for example, four times daily).  The medication prescription may state that the medication is to be given after a certain amount of time has passed (for example, every 6 hours).  Note that the two intervals stated as examples are really the same.  Every 6 hours is the same as 4 times daily.

The nurse will calculate the amount to give for the entire day then divide the ordered number of dose.  Note: do not round the quantity of medication until the amount per dose is calculated.  Rounding before the amount per dose is calculated may result in an incorrect quantity per dose and in total.

If the problem does not state divided doses, the prescription is written for one dose.

# Problem Type 1 – Medication Given in Divided Doses

Example 1:

A healthcare provider has prescribed lorazepam 4 mg IM in six divided doses.  How much will the nurse give per dose?

Here’s the problem set up in the dimensional analysis format:

SF = 4 mg

AU = ml per dose

Equivalents:

4 mg = 1 ml  (see the drug label)

Equation for the dose in ml:

Example 2:

A healthcare provider has prescribed ondansetron HCl 24 mg orally daily in divided doses every 8 hours.  How many ml will the nurse give per dose?

SF = 24 mg

AU = ml per dose

Equivalents:

4 mg = 5 ml (see label)

Equation for the dose in ml:

#### Reconstitution of Powdered Medications

Some medication is supplied in a powdered form and must mixed with a specified liquid (diluent) before it is used.

http://www.austincc.edu/rxsucces/pdf/reconstructionpdf.pdf     Retrieved 1/27/19

Instructions for reconstitution will be provided on the drug label or package insert.  Be certain that you are using the correct diluent in the correct quantity for each medication,

In the label above, 2.7 ml of sterile water is the specified diluent for oxacillin sodium.  First inject 2.7 ml of air into the vial of sterile water to compensate for the volume of fluid to be removed.  Then withdraw 2.7 ml of sterile water to use as the diluent.

Inject the sterile water into the vial of powdered medication.  Swirl or rotate to mix as directed.  The medication is then ready to use.  The date and time of reconstitution and the nurse’s name must be recorded on the vial of medication because medication must not be used after its shelf life has expired.

The dosage strength of the reconstituted medication will be specified on the label.  The dosage strength of the reconstituted medication is the strength the nurse will use in calculating the amount of medication to give the client according to the healthcare provider’s prescription.

Some medications will give a range of diluent amounts, each resulting in a different dosage strength of the medication.  See the right side of the medication label above. The amount of diluent to use will be selected based on the dose to be given.  The amount of diluent used must be recorded so the dosage strength will be clear to anyone using the reconstituted medication.

Recording the amount of diluent used will prevent possible medication errors due to a misunderstanding of the medication dosage strength.

# Problem Type 2 – Reconstitution of Powdered Medications

Example 1:

The healthcare provider has prescribed fluconazole oral suspension 200 mg by mouth  daily for 7 days.  How many ml will the nurse give to the client per dose?

SF = 200 mg

AU = ml

Equivalents:

Note - The nurse has added 24 ml of distilled or purified water to the powdered medication for reconstitution, but this amount is not relevant in calculating the dose to give.  The label states the drug strength after reconstitution.

40 mg = 1 ml

Equation for the dose in ml:

Example 2:

The healthcare provider has prescribed 2 g streptomycin IM daily divided every 8 hours for your client.  Your charge nurse recommends that you use 1.8 ml of sterile water to reconstitute the medication to reduce the volume to be injected.  How many ml will you give per dose?

SF = 2 g

AU = ml per dose

Equivalents:

1 g = 1000 mg

400 mg = 1 ml  (from the reconstitution directions on the label)

You do not use the 1.8 ml of diluent added in your calculations, but you need this information to find the 400 mg per ml after reconstitution from the drug label.

Equation for the dose in ml:

One day = 24 hours

Every 8 hours  =  3 doses per day

## Practice Problems

Module 6 Practice Problems

Use the Adderall XR label above for problems 1 and 2.

1.  You are working with a young adult client.  The healthcare provider has prescribed Adderall XR capsules 60 mg daily in two divided doses at 0600 and 1200.  How many capsules will the client take per dose?

1. Your client taking Adderall XR has a new prescription.  The client will now take the 45 mg of the medication daily in three divided doses at 0600, 1100, and 1400.   How many capsules will the client take per dose?

Use the Benadryl label above for problems 3 and 4.

1. A nurse is working in a pediatric clinic.  A client who is 11 years old has a new prescription for diphenhydramine HCl elixir 40 mg by mouth daily in four divided doses.  How many mL will the client take per dose?

1. The prescription for the 11-year-old client in problem 3 has changed.  The client will now take diphenhydramine HCl elixir 60 mg by mouth daily in four divided doses.  How many mL will the client take per dose?

Use the label for Dilantin-125 above for problems 5 and 6.

1. A nurse is working with a 16-year-old client.  The healthcare provider has written a prescription for phenytoin oral suspension 7 mg/kg/day by mouth in three divided doses.  The client weighs 125 pounds.  How many mL will the client take per dose?

1.  The nurse has another client who is 12 years old.  This client has a prescription for phenytoin oral suspension 6 mg/kg/day by mouth in three divided doses.  The client weighs 55 kg.  How many mL will the client take per dose?

Use the label for ondansetron injection above for problem 7.

1. A nurse working on a hospital medical client has a client on IV fluids.  The healthcare provider has written an additional order for ondansetron 24 mg IV in three divided doses.  How many mL of ondansetron will the nurse give per dose?

Use the tobramycin label above for problem 8.

1. The hospitalized client from problem 7 above has a new prescription for tobramycin 1.5 mg/kg/day IV every eight hours in divided doses.  The client weighs 180 pounds.  How many mL will the nurse give per dose?

Use the Phenergan injection label above for problems 9 and 10.

1. Your adult client has a prescription for promethazine HCl injection IM 100 mg daily in three divided doses.  How many mL will you give per dose?

1. Your client’s prescription has changed to promethazine HCl injection IM 80 mg daily in three divided doses.  How many mL will you give per dose?

Use the label above for amoxicillin oral suspension for problems 11 and 12.

1.  Your adult client has a prescription for amoxicillin oral suspension 1300 mg by mouth daily in three divided doses.  What diluent will you mix with the powder for reconstitution?  How much diluent will you use?  How many mL of the reconstituted medication will you give per dose?

1. Another of your adult clients has a prescription for amoxicillin oral suspension 400 mg by mouth every eight hours.   How many mL of the reconstituted medication will you give per dose?

Use the Zithromax label above for problems 13 and 14.

1.    Your hospitalized adult client has a prescription for azithromycin 500 mg IV daily for two days.  How many mL will you give per dose?

1.  Your client’s prescription has changed to azithromycin 450 mg IV daily.  How many mL will you give per dose?

Use the label for Ryanodex for questions 15 and 16.

1. A nurse is caring for a hospitalized adult client who weighs 150 pounds.  The healthcare provider has written a prescription for dantrolene sodium IV 1 mg/kg every four hours.  How many mL will the nurse give per dose?  What is the diluent used to reconstitute the medication?

1. The client in question 15 has a new prescription.  The prescription for dantrolene sodium IV has changed to 2.5 mg/kg IV every six hours.  How many mL will the nurse give per dose?

Use the Pfizerpen label above for questions 17 through 20.

A nurse is working on a hospital medical unit and has several clients with prescriptions for penicillin G potassium IV.

1.  Client A has a prescription for 10 million units of penicillin G potassium IV daily in four divided doses.  The bottle of penicillin G potassium available on the unit was reconstituted using 75 mL of sterile water.  How many mL will the nurse give per dose?

1.   Client B has a prescription for 20 million units of penicillin G potassium IV daily in six divided doses.  The nurse uses 33 mL of sterile water to reconstitute a new bottle of  penicillin G potassium.   How many mL will the nurse give per dose?

1.   Client C has a prescription for 30 million units of penicillin G potassium IV daily in six divided doses.  The nurse uses 75 mL of sterile water to reconstitute a new bottle of  penicillin G potassium.   How many mL will the nurse give per dose?

1. The prescription for client C has changed to 25 million units of penicillin G potassium IV daily in four divided doses.  The nurse still has medication in the bottle of penicillin G potassium reconstituted with 75 mL of sterile water.  How many mL will the nurse give per dose?

### Module 6 Answers to Practice Problems

Why do you often need the client’s age?  Doses of the medication often differ by age, especially when the clients are children or teens.

1. You are working with a young adult client.  The healthcare provider has prescribed Adderall XR capsules 60 mg daily in two divided doses at 0600 and 1200.  How many capsules will the client take per dose?

SF = 60 mg

AU = capsules

Equivalents:

1 capsule = 15 mg

2 doses

Equation for the dose in caps:

1. Your client taking Adderall XR has a new prescription.  The client will now take the 45 mg of the medication daily in three divided doses at 0600, 1100, and 1400.   How many capsules will the client take per dose?

SF = 45 mg

AU = capsules

Equivalents:

1 capsule = 15 mg

3 doses

Equation for the dose in caps:

1. A nurse is working in a pediatric clinic.  A client who is 11 years old has a new prescription for diphenhydramine HCl elixir 40 mg by mouth daily in four divided doses.  How many mL will the client take per dose?

SF = 40 mg

AU = mL

Equivalents:

5 mL = 12.5 mg

4 doses

Equation for the dose in mL:

1. The prescription for the 11-year-old client in problem 3 has changed.  The client will now take diphenhydramine HCl elixir 60 mg by mouth daily in four divided doses.  How many mL will the client take per dose?

SF = 60 mg

AU = mL

Equivalents:

5 mL = 12.5 mg

4 doses

Equation for the dose in mL

The Dilantin questions:

1. A nurse is working with a 16-year-old client.  The healthcare provider has written a prescription for phenytoin oral suspension 7 mg/kg/day by mouth in three divided doses.  The client weighs 125 pounds.  How many ml will the client take per dose?

Remember: In weight-based problems, the client’s weight is the starting factor.  If the weight is in pounds and the prescription is in kg, a weight conversion should be built into your equation.

SF = 125 lbs

AU = mL

Equivalents:

5 mL = 125 mg

1 kg = 2.2 lbs

3 doses

Equation for the dose in mL:

In this problem there is no difference in the answer, but remember to not round your answer to the appropriate number of decimal places until you reach your final answer.

1. The nurse has another client who is 12 years old.  This client has a prescription for phenytoin oral suspension 6 mg/kg/day by mouth in three divided doses.   The client weighs 55 kg.  How many mL will the client take per dose?

SF = 55 kg

AU = mL

Equivalents:

5 mL = 125 mg

3 doses

Equation for the dose in mL:

The Ondansetron question:

1. A nurse working on a hospital medical client has a client on IV fluids.  The healthcare provider has written an additional order for ondansetron 24 mg IV in three divided doses.  How many mL of ondansetron will the nurse give per dose?

SF = 24 mg

AU = mL

Equivalents:

1 mL = 2 mg

3 doses

Equation for the dose in mL:

The Tobramycin question:

1. The hospitalized client from problem 7 above has a new prescription for tobramycin 1.5 mg/kg/day IV every eight hours in divided doses.  The client weighs 180 pounds.  How many mL will the nurse give per dose?

SF = 180 lbs

AU = mL

Equivalents:

1 mL = 40 mg

1 kg = 2.2 lbs

3 doses

Equation for the dose in mL:

The Phenergan questions:

1. Your adult client has a prescription for promethazine HCl injection IM 100 mg daily in three divided doses.  How many mL will you give per dose?

SF = 100 mg

AU = ml

Equivalents:

1 mL = 25 mg

3 doses

Equation for the dose in mL:

1.  Your client’s prescription has changed to promethazine HCl injection IM 80 mg daily in three divided doses.  How many mL will you give per dose?

SF = 80 mg

AU = mL

Equivalents:

1 mL = 25 mg

3 doses

Equation for the dose in mL:

The Amoxicillin questions:

1. Your adult client has a prescription for amoxicillin oral suspension 1300 mg by mouth daily in three divided doses.  What diluent will you mix with the powder for reconstitution?  How much diluent will you use?  How many mL of the reconstituted medication will you give per dose?

The diluent is water. You are to add 59 mL of water to the powder.

SF = 1300 mg

AU = mL

Equivalents:

5 mL = 250 mg

3 doses

Equation for dose in mL:

1.  Another of your adult clients has a prescription for amoxicillin oral suspension 400 mg by mouth every eight hours.   How many mL of the reconstituted medication will you give per dose?

SF = 400 mg

AU = mL

Equivalents:

5 mL = 250 mg

Equation for dose in mL:

This problem is not written for divided doses.  The dose stated will be given every 8 hours.

The Zithromax questions:

1. Your hospitalized adult client has a prescription for azithromycin 500 mg IV daily for two days.  How many mL will you give per dose?

SF = 500 mg

AU = mL

Equivalents:

1 mL = 100 mg

Equation:

This problem is not written for divided doses.  The dose stated will be given every day.

The medication label states to dilute further before use.  That would be done after the correct dose is drawn from the vial.

1. Your client’s prescription has changed to azithromycin 450 mg IV daily.  How many mL will you give per dose?

SF = 450 mg

AU = mL

Equivalents:

1 mL = 100 mg

Equation:

This problem is not written for divided doses.  The dose stated will be given every day.

The medication label states to dilute further before use.  That would be done after the correct dose is drawn from the vial.

The Ryanodex questions:

1. A nurse is caring for a hospitalized adult client who weighs 150 pounds.  The healthcare provider has written a prescription for dantrolene sodium IV 1 mg/kg every four hours.  How many mL will the nurse give per dose?  What is the diluent used to reconstitute the medication?

SF = 150 lbs

AU = mL per dose

Equivalents:

1 mL = 50 mg

1 kg = 2.2 lbs

Equation:

This problem is not written for divided doses.

The diluent is sterile water for injection (supplied in a vial).

1. The client in question 15 has a new prescription.  The prescription for dantrolene sodium IV has changed to 2.5 mg/kg IV every six hours.  How many mL will the nurse give per dose?

SF = 150 lbs

AU = mL per dose

Equivalents:

1 mL = 50 mg

1 kg = 2.2 lbs

Equation:

This problem is not written for divided doses.

The Pfizerpen questions:

1. Client A has a prescription for 10 million units of penicillin G potassium IV daily in four divided doses.  The bottle of penicillin G potassium  available on the unit was reconstituted using 75 ml of sterile water.  How many ml will the nurse give per dose?

SF = 10,000,000  units

AU = mL

Equivalents:

1 mL = 250,000 units

4 doses

Equation:

1. Client B has a prescription for 20 million units of penicillin G potassium IV daily in six divided doses.  The nurse uses 33 mL of sterile water to reconstitute a new bottle of  penicillin G potassium.   How many mL will the nurse give per dose?

SF = 20,000,000  units

AU = mL

Equivalents:

1 mL = 500,000 units (see label – amount of diluent has changed)

6 doses

Equation:

1. Client C has a prescription for 30 million units of penicillin G potassium IV daily in six divided doses.  The nurse uses 75 mL of sterile water to reconstitute a new bottle of  penicillin G potassium.   How many mL will the nurse give per dose?

SF = 30,000,000  units

AU = mL

Equivalents:

1 mL = 250,000 units (see label – diluent has changed again)

6 doses

Equation for the dose in ml:

1. The prescription for client C has changed to 25 million units of penicillin G potassium IV daily in four divided doses.  The nurse still has medication in the bottle of penicillin G potassium reconstituted with 75 ml of sterile water.  How many mL will the nurse give per dose?

SF = 25,000,000  units

AU = mL

Equivalents:

1 mL = 250,000 units

4 doses

Equation:

Floyd Library - 706.295.6318 | Heritage Hall - 706.295.6321 | Cartersville Library - 678.872.8400 | Marietta - 678.872.8501 | Paulding Library - 678.946.1007